1. Field of the Invention
The invention relates to fumed silanized silica, to a process for preparing it and to its use.
Fumed silica (pyrogenically prepared silicon dioxide) is known from Ullmanns Enzyklopädie der technischen Chemie, Volume 21, page 464 (1982).
2. Description of Related Art
It is prepared by burning a vaporizable silicon compound, such as silicon tetrachloride, for example, in a mixture with hydrogen and oxygen.
The comminution of materials to form coarse powders (50-500 μm), fine powders (5-50 μm) and even greater finenesses (less than 5 μm) is common and widespread practice. For all comminution tasks there is a multiplicity of technical and industrial equipment offered and operated, all adapted to the particular circumstances of the specific tasks. A good overview of the comminution problems and of the diverse machines is given in Ullmanns Enzyklopädie der technischen Chemie, 3rd Edition, Volume 1, pages 616 to 638.
For fumed silica the average primary particle diameters are notably lower (5-50 nm) than can be obtained by mechanical comminution.
The primary particles and aggregates of the fumed silica with a surface area of 200 m2/g can be visualized in the electron microscope.
The primary particles and aggregates of a fumed silica agglomerate to form larger assemblies whose size is generally in inverse proportion to the primary particle size or in proportion to the specific surface area. The agglomerate size also increases in line with the extent to which the fumed silica is compacted.
The binding forces holding these agglomerates together are relatively weak. Nevertheless, when these agglomerates are incorporated into and broken down in a liquid system for the purpose of homogeneous distribution of the primary particles and aggregates, or particles with a low degree of agglomeration, a certain shearing energy is required. Depending on the particular field of application, dispersing is carried out using any of a very wide variety of mixing devices, with determining factors for the selection being not only the viscosity and polarity of the system but also the agglomerate strength and the desired homogeneity.
With simple agitator mechanisms, such as paddle stirrers, it is usually not possible to carry out to satisfaction the direct incorporation of small amounts of silicas, especially not when the systems in question are of low viscosity. However, paint and varnish makers, and also those who carry out processing, have an interest in obtaining performance-optimum distribution of the silicas, which are used predominantly as thickeners and thixotropic agents, by means of very simple devices, with a very low energy input and in a very short time.
In the case of paddle-stirrer dispersing, the coarse silica agglomerates are not sufficiently comminuted and hence are able to make only a small contribution to raising viscosities and thixotropy. The data relate to a UP resin (unsaturated polyester resin) as dispersion medium.
Reducing the agglomerate size by dispersing outside a liquid system, in other words, in practice, in the air, or by grinding in the conventional sense, is possible only to a limited extent, since in the case of material with a given propensity to agglomerate the comminution is followed immediately by the re-establishment of the old agglomeration state. This effect comes about no later than after recompaction of the material which as a result of the mechanical intervention has undergone a high degree of loosening and which in this form is not amenable to dispatch and storage. The storage time as well would have the effect of renewed agglomerate enlargement.
A value taken as a dimensional number and evaluation variable for the stage of distribution of a dispersible silica and maximum agglomerate size of the dispersion (granularity) is the so-called grindometer value to DIN 53203.
A known procedure is to render fumed silica hydrophobic, to grind it in a pinned-disc mill and then to classify it (US 2004/0110077 A1).
This known silica is used as an external additive in toner mixtures.
Hydrophilic fumed silica with a BET surface area possesses a grindometer value as determined in UP resin (unsaturated polyester resin Ludepal P6 from BASF, 2% dispersion) in accordance with the DIN specification, of 50 to 60.
If this fumed silica is also relatively highly compacted (100 to 120 g/l), the grindometer value as well is also significantly higher, specifically more than 100, whereby necessitating an additional, not inconsiderable energy as a thickener and thixotropic agent.
A known procedure is to grind a highly dispersed silica having a surface area of approximately 300 m2/g in a pinned-disc mill.
The grindometer value achieved is initially, for the uncompacted silica, 25.
If this silica is compacted to 50 g/l, the grindometer value rises to 30, and in the case of further compaction to 75 g/l it rises to as far as about 40.
In the case of storage over a period of three months, the ground silica, not modified but compacted to 50 g/l, has a grindometer value of 50 to 60.
Reagglomeration can only be prevented in accordance with the prior art if the hydrophilic silica is mixed with 3% by weight of a hydrophobic silica and if this mixture is ground by means of an air-jet mill or a pinned-disc mill (EP 0 076 377 B1).
In this case, for a fumed silica having a BET surface area of 200 m2/g, even after compaction to 73 or 107 g/l, a grindometer value of 35 is achieved.
For a fumed silica having a BET surface area of 300 m2/g, the addition of hydrophobic silica prior to grinding produces a grindometer value of 10 for a tamped density of 28.1 g/l and of 15 to 20 for a tamped density of 50 g/l.
The known fumed silicas have the disadvantage that they contain an unwanted fraction of hydrophobic silica.